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For scientists, our earliest ancestor wasn’t Adam or Eve but Luca. Luca didn’t look anything like us – it was a single-celled bacterium-like organism. A recent study by a team of scientists based in the UK has delivered rather shocking news about this illustrious forebear. Despite having lived almost as far back as seems possible, Luca was surprisingly similar to modern bacteria – and what’s more, it apparently lived in a thriving community of other organisms that have left no trace on Earth today.
Luca – short for the last universal common ancestor, the progenitor of all known life on Earth – seems to have been born 4.2bn years ago. Back then our planet was no Eden but something of a hell on Earth: a seething mass of volcanoes pummelled by giant meteorites, and having recovered from a cosmic collision that blasted the world apart and created the moon from some of the fragments. There is good reason why the geological aeon before 4bn years ago is called the Hadean, after the Greek god of the underworld Hades.
If Luca really was so ancient, yet already so sophisticated and embedded in a whole ecosystem, there’s a startling implication that goes far beyond an understanding of our own origins. It suggests that life must have got started on Earth pretty much as soon as it possibly could have done. Which in turn implies that, given the right conditions and ingredients, life might not be an extremely rare and unlikely accident, as some scientists have believed, but rather, almost an inevitability, and therefore likely to be abundant in the universe.
Luca’s existence is a corollary of Darwinian evolutionary theory, whereby all living organisms from microbes to whales descended from earlier ones in one vast tree of life. We humans share a common ancestor with chimpanzees and bonobos that lived about 6-8m years ago. All monkeys and apes are thought to have branched from a common ancestor about 25m years ago. Keep going back down the tree far enough and you will find a common ancestor of all mammals, then all vertebrate organisms, and so on.
Luca represents the point where the three domains of all life – eukarya (including animals, plants and fungi), bacteria, and archaea (another kind of microbe) – converge to a single stem. When this happened has been the subject of debate for decades. Luca was once thought to have lived about 3.5-3.8bn years ago, comfortably outside the Hadean. But recent studies have pushed it ever further back in time.
It might seem unlikely that we could know anything at all about Luca. There are no fossil records of life that old, and very few rocks remain unchanged from that time to hold clues about what the Earth was like. But scientists can make deductions about such early organisms using the technique of molecular phylogenetics. By comparing the genetic sequences of organisms alive today, they can figure out from their similarities and differences the order in which the various species split off as separate branches of the evolutionary tree, as well as what genes their common ancestors might have possessed. And if they know the rate at which mutations in genes create such differences, this provides a “molecular clock” that can furnish estimates of when the branching happened.
Stretching such analyses all the way back to Luca on the very early Earth, based only on genetic information about organisms alive today, is a tall order. The reconstructed genomes of such ancient ancestral organisms are simply best guesses, and patchy even then. That’s why the age and the nature of Luca have been contentious. But the task becomes more reliable as we gather ever more genetic information about modern organisms.
Last July, a team led by researchers at Bristol University reported on a state-of-the-art molecular phylogenetic study that pointed to the conclusion that Luca lived 4.2bn years ago, give or take 100m years. That’s within the range, but towards the most ancient end, of some earlier estimates.
This Hadean Earth had no breathable air: oxygen today is produced by photosynthesis by plants and bacteria, which began much later. The atmosphere contained lots of carbon dioxide, says Earth system scientist Tim Lenton of Exeter University, a co-author of the new study – and as a result, “the sky may have been less blue than it is today”. It might even have had an orange hint from a haze of methane.
Earth was then a water world, covered entirely in ocean with just a few volcanic islands poking above the waves. What’s more, says marine microbiologist Rika Anderson of Carleton College in Northfield, Minnesota, “the Earth was spinning faster on its axis, so days were 12 hours long. And the moon was closer than it is now, so tides were stronger.”
How did Luca sustain itself? The phylogenetic analysis shows that it had all the molecular machinery – the protein enzymes – it needed to feed itself from simple molecules in its surroundings, specifically carbon dioxide and hydrogen. Living at the sea surface, it could have got both of those from the atmosphere. Alternatively, Luca could have gleaned them from so-called hydrothermal vents in the deep sea, where volcanic heat sends hot water within fissures in the rock streaming out of chimney-like geological formations, enriched in minerals and dissolved gases. Some researchers think that life itself began at such deep vents, protected from the scourge of meteorite bombardment.
That would make Luca a chemoautotroph: an organism able to make the chemicals it needs from simple ones formed in geological processes. But it could also have been a heterotroph, dependent on chemicals made in metabolic reactions by other organisms in the ecosystem. At any rate, the new study shows that Luca had quite a complex set of metabolic enzyme machinery: it wasn’t a rough first draft of life, but already a pretty sophisticated and refined piece of work, suggesting that it had already been evolving for ages.
Luca probably “did not live alone”, says Lenton. By making complex organic molecules, it would have created an environment where other heterotrophic organisms could thrive, perhaps some by gobbling up Luca itself. “It would have created niches for other bugs to make a living based on its waste products,” says palaeobiologist Philip Donoghue, one of the leaders of the Bristol team.
The researchers think Luca’s neighbours might have included organisms that, like many microbes today (including those in the gut), generated methane (CH4), thereby returning carbon and hydrogen into the atmosphere. “That creates a recycling loop that makes everyone more productive,” says Lenton. If Luca did indeed live at a vent, says Anderson, some members of its community might have used sulphur or iron in the vent fluids as their fuel. A recent study by researchers at the University of Arizona in Tucson supports that idea, finding that sulphur-containing and metal-binding amino acids were among the first to be used by Luca and its ancestors for making proteins.
The researchers also find that Luca had a kind of immune system to protect it from viral infection. Some bacteria today have a defence system called CRISPR-Cas, which can stitch pieces of viral genomes into the DNA of the host, creating a molecular memory of past infection to accelerate a defensive response, much as our own immune systems does. Luca’s reconstructed genome seems to include instructions for a CRISPR-Cas-like apparatus, suggesting that viruses were rife – and a potential problem – in its ecosystem.
This is perhaps no surprise, for some researchers now believe that viruses – parasites that hijack the machinery of their host cells to replicate themselves – are an inevitable outcome of how life works by DNA replication. “I tend to think of viruses as being universal to life,” says Anderson. But she adds that she does not imagine viruses back then looked like viruses today, “so I was a little surprised to see that a CRISPR system existed in Luca”. It’s a sophisticated bit of kit for such an ancient organism.
But a virus-ridden world was not necessarily a bad thing. On the contrary, viruses might have helped create such a rich ecology on the early Earth. Because they can paste new genes into host DNA, viruses can act as vehicles for carrying genetic material from one organism to another by “horizontal gene transfer”: a way for organisms to share genes without them being directly related. Luca’s ecosystem might have been a hotbed of virally assisted gene-sharing, inducing more diversity than could have arisen by conventional Darwinian evolution through descent and natural selection.
In this view, the early tree of life was not so much a tree at all, but a densely connected web. (In some ways it still is.) And if it seems a little sad that, from those rich ancient biospheres, only Luca has left modern descendants, nonetheless horizontal gene transfer might have implanted little snippets of this lost genetic world within Luca itself.
Anderson says that Luca’s venerable age needs more confirmation from other sources, such as the geological record. And Donoghue admits that “I don’t think we can say anything about Luca for sure, other than that it existed”. But this study is surely not the last word. “I think there’s more to come, for sure,” says Anderson. “Our tools and data keep getting better and better, and geochemists are using more clever ways to look back in time to infer what the early Earth and its inhabitants were like.”
If it holds up, the antiquity of Luca seems to overturn some earlier arguments for why the universe is largely lifeless. “These have been based on the evidence that it took about a billion years for life to emerge on Earth, meaning that these early steps were hard and/or unlikely,” says Donoghue. But a 4.2bn-year-old and already rather highly evolved Luca, Lenton says, “tells us that [starting] life is not that hard. It can start all over the place on planets with liquid water, possibly including early Mars or even early Venus.”
Astronomical searches for planets around other stars have suggested that Earth-like planets are not all that uncommon. Still, there might have been special features of our planet, Anderson cautions, that made it particularly amenable to life, such as a magnetic field to shield us from solar radiation, a large neighbouring planet (Jupiter) to sweep up stray asteroids, and a moon to create tides.
Besides, says Lenton, the challenge is not just to start a biosphere but to keep it going: “to have life affect its planetary environment in a way that helps keep it habitable”, as argued by the Gaia theory of the late scientist and inventor James Lovelock, with whom Lenton worked closely. He believes Gaia-style maintenance of a biosphere should be quite common, once it has started. “I am therefore predicting that other biospheres are out there waiting to be discovered.”
